Aqueous sodium nitrate solutions are among those used in industry to electrochemically machine articles made from stainless steel alloys. As known, stainless steel alloys contain varying amounts of chromium. Thus, when stainless steel alloys are electrochemically machined, part of the chromium metal in the alloy is converted to hexavalent chromium and remains in the aqueous sodium nitrate solution.
Hexavalent chromium accumulates in sodium nitrate solutions and is highly soluble over a wide range of pH. Since electrochemical machining (ECM) solutions are continuously reused, the build-up of hexavalent chromium poses potential environmental concerns, as well as possible health hazards to machine operators. In order to efficiently and safely reuse the aqueous sodium nitrate solutions, there is a need to develop a method that reduces hexavalent chromium to less toxic trivalent chromium, and removes chromium from ECM solutions without introducing unwanted ions.
A well-known method for removing chromium from electrolytic solutions involves ferrous ion reduction of hexavalent chromium to trivalent chromium. Typically, ferrous sulfate or ferrous chloride is employed as the reducing agent. The result is a mixture of iron hydroxides and chromic hydroxides which are precipitated under alkaline conditions. One drawback to ferrous ion reduction is that the sulfate or chloride ions remain in solution as unwanted ions. If ferrous nitrate were commercially available, it could be used and would not introduce unwanted ions to the solution.
Copending commonly owned application Ser. No. 08/080549 discloses a method to remove unwanted sulfate ions from chromium-contaminated sodium nitrate solutions. Along with ferrous sulfate, barium nitrate is added to the machining solution so that the sulfate is precipitated as insoluble barium sulfate. Sodium hydroxide is then utilized to raise the pH and complete the precipitation of the iron and chromium species as hydroxides. Unwanted sulfate ions are removed and the machining solution is suitable for reuse.
To avoid mixing reagents in and adjusting the pH of the entire volume of an electrochemical machining solution, there is a need for an inexpensive and efficient method of introducing a concentrated ferrous reducing agent to the solution without introducing unwanted ions to the solution.